A wide bandgap semiconductor such as SiC (silicon carbide) or GaN (gallium nitride) has better physical values than Si (silicon), in terms of dielectric breakdown strength, electron saturation speed, thermal conductivity, and the like. Accordingly, such materials are expected as materials for next-generation semiconductor devices, particularly for power semiconductor devices.
A power device that performs switching operation, such as a transistor, preferably has a sufficiently high threshold voltage for security reasons. However, in a transistor using a wide bandgap semiconductor, it is difficult to realize a sufficiently high threshold voltage.
In a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) using SiC, for example, the threshold voltage becomes lower due to the influence of interface levels existing between the semiconductor and the gate insulating film. In a HEMT (High Electron Mobility Transistor) using a GaN-based semiconductor, for example, there is the problem of “normally-on operation” in which conduction is achieved without application of voltage to the gate electrode, due to the existence of a two-dimensional electron gas under the gate electrode.
Particularly, in a MOSFET using SiC, if interface nitriding is performed so as to increase mobility, the threshold voltage becomes lower. This trade-off is more prominent in a MOSFET having a high carrier mobility.